This invention relates to data communications, and more particularly to data communications using modems having digital identification information capability.
Data communications often involves the use of modulator/demodulators (modems) to communicate over a voiceband public network system. Until recently, modem connections between such network systems have had two analog links. For example, as shown in FIG. 1, two computers 1, 2 may each have a modem link 3, 4 (land-line or wireless) to the public switched telephone network (PSTN) 5. In such a configuration, each modem conforms to the analog signaling standards of the network. One consequence is that receive and transmit signals are combined on some segments of the analog system. For example, FIG. 2 shows two modems A, B, each having a receiver Rx and a transmitter Tx coupled by means of a corresponding 2-wire to 4-wire hybrid 20A, 20B to 2-wire analog segments 22A, 22B. Elsewhere in the system, within the PSTN 5, the 2-wire segments 22A, 22B are coupled to corresponding 2-wire to 4-wire hybrids 24A, 24B that are intercoupled by a 4-wire segment 26, which may be analog or digital.
A characteristic of such networks is that the hybrids cause echos due to impedance mismatches at the hybrids in the network, and such echoes interfere with communication. The echo experienced by a modem is greatest from the nearest hybrid, and decreases with distance due to the attenuation of the channel. For example, modem A in FIG. 2 would generally experience the largest echo from its own internal hybrid 20A, and a lesser echo from the near network hybrid 24A (because the echo from hybrid 24A is attenuated by the loss of the analog segment 22A). There are also smaller echos from the far network hybrid 24B and the internal hybrid 20B of modem B, but these echos can be treated as one echo. The time between the near and far echoes can be large (e.g., on the order of 2 seconds). Accordingly, most modern modems, such as those that conform to the ITU V.34 standard, have been designed with two echo canceling circuits, one for each of the near and far echo sources. (Occasionally hybrids exist in the middle of these two common sources, and cause a third echo. Most modems cannot deal with such intermediate echos, which contribute to overall noise in the link and reduce connect speeds.) A standard, V.90, has recently been established for a pulse-code modulation (PCM) voiceband modem system. A PCM modem system is different than traditional voiceband modems in that a PCM modem system takes advantage of a frequently-encountered network topology to attain higher data rates. PCM modem communications requires at most one analog component. Such a topology, shown in FIG. 3, includes an Analog Modem coupled to an analog segment 30. Transmit and receive signals are combined on the analog segment 30. The analog segment 30 is coupled to a digital segment 32 through an analog/digital interface 34 that is part of the PSTN 5. Under current standards, the digital segment 32 uses standard transmission facilities (e.g., T1/E1 transmission systems) and provides separate transmit and receive signal paths to a separate receiver Rx and transmitter Tx within a PCM Modem.
The analog/digital interface 34 comprises a combination hybrid and PCM coder/decoder (codec). The hybrid separates the analog transmit and receive signals. The PCM codec provides analog-to-digital (A/D) and digital-to-analog (D/A) conversion and PCM encoding and decoding.
Because the PCM Modem is connected directly to the digital segment 32, the digital-to-analog direction of transmission suffers fewer impairments than a standard analog-to-analog modem connection, as in FIG. 2. The most significant improvement comes from lack of quantization noise. As a result, speeds nearing the 64 Kbps capacity of a digital telephone connection may be achieved in the digital-to-analog direction. However, in the analog-to-digital direction, quantization noise is still a problem due to the PCM codec within the analog/digital interface 34.
In order for modem standards to progress, modems must be able to identify their capabilities to each other. Until recently, there has been no way of signaling to an analog modem that the modem it is talking to is digitally connected. However, recently many modems have begun using an ITU standard, V.8 and/or V.8bis, for identification purposes. This standard includes a xe2x80x9ccapabilitiesxe2x80x9d information exchange. The ITU has recently agreed to extend the information exchanged to include an indication of a capability for PCM modem operation (PCM capability), and whether a modem has a digital or analog connection to the network (connection type). Under this standard, two analog modems incapable of PCM modem operation would fall back to a traditional analog modem standard, but a PCM digital modem and an analog PCM modem would try to connect as a PCM modem pair.
Another enhancement to the PCM modem standard known as xe2x80x9cV.admxe2x80x9d encompasses coupling two PCM modems together by a completely digital link through the PSTN 5. For example, as shown in FIG. 4, a PCM Modem A is coupled to a PCM Modem B by means of separate digital data channels within corresponding digital segments 40A, 40B (the dotted lines withing the PSTN 5 indicate that the connection type within the PSTN is unknown). Such a digital segment may conform, for example, to the ISDN standard.
The invention includes improved methods of data communication for modems having digital identification information capability. More particularly, the invention is based on the realization that the ability to signal connection type information between modems permits new uses of existing analog modem configurations.
More particularly, the invention permits efficient utilization of an analog modem when communicating with a host modem, the analog modem having a far echo canceling capability, and includes: determining if the host modem has digital communication capability; communicating between the analog modem and the host modem using analog communication processing if the host modem is determined not to have digital communication capability; communicating between the analog modem and the host modem using a modified communication processing protocol if the host modem is determined to have digital communication capability, the modified communication processing protocol including: terminating any use of the far echo canceling capability of the analog modem for far echo cancellation; optionally applying the far echo canceling capability of the analog modem to improve near echo cancellation for the analog modem; and optionally applying the far echo canceling capability of the analog modem to train for cancellation of any intermediate echo that may affect the analog modem.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.